Crack Paths 2012

Rolled bolts exhibit an increase in fatigue life compared to those machined, due to the

compressive residual stresses generated upon notch roots [1,7]. The fatigue strength of

machined bolted elements is influenced to the greatest degree by the wearing of the tool and

the cutting speed, whereas the cutting method and the radial feed are of lesser importance [8].

In groove-rolled products, tools’s high penetration rate increases the fatigue strength, due to

the appearance of greater compression axial residual stresses and strain hardening [9].

The rolling of steel bolts after annealing, compared to most commonprocedure (machining)

that is the usual one, increases the fatigue life remarkably for low minimumstresses [10,11]. On

the contrary, for high minimumstresses there are barely any changes if the bolts are of normal

pitch [10] and the increase is muchlower if the bolts are of fine pitch [11].

E X P E R I M E N TPARLO C E D U R E

The specimens were commercial M10x200bolts with 8.8 grade (DIN 931), made of

blued steel, and without head in a cylindrical piece (with machined inner thread) of

16 m mof diameter, 150 m mlength and same material as the bolt (Fig. 1).

M 1 0

1 6

20

36

Figure 1. Bolted joint.

The mechanical behaviour of steel in bolts has been characterized by simple

tension tests, with specimens obtained from the body of the bolts, using a crosshead

speed of 2 mm/min.

Tests with axial monotonic stress controlling the displacement (crosshead speed of

2 mm/min)were carried out on the specimens with the bolted joint until fracture. For the

fatigue characterization, load control tests were performed (constant stress range ),

with the form of a sinusoidal wave, frequency of 10 Hz and different values for the

R-ratio (0, 0.25 and 0.50). These tests were carried out until fracture or until they reached

106 cycles. Somespecimens were subjected to a tensile preload of 80%of the theoretical

yield strength of the bolt material, in order to study howit affected the fatigue life.

E X P E R I M E N TR AE SLU L T S

Material Characterization

The steel used for this study, after its metallographic preparation and being etched with

4 %Nital, presented a ferritic-pearlitic

microstructure, as displayed in Fig. 2.

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